1. Field of the Invention
This invention is directed to an improved radial tires for use in high pressure, heavy duty trucks and buses, and more particularly to an improvement in durability of the carcass plies therein.
2. Prior Art
Carcass plies of conventional radial tires for use in high pressure, heavy duty trucks and buses are normally made of steel cords which frequently rust undesirably. In other words, there are many faults created. Steel cords rust due to penetration of moisture through injuries to the tire surface. In long-term service, air which is within and inflates the tire can penetrate into the carcass ply layer through the internal wall of the tire and thereby oxidize the steel cords together with the moisture in the air so as to cause rusting. Therefore the conventional radial tires have the disadvantages that there is possible a reduction in strength of the steel cords and an adhesion of the carcass to the coating rubber.
Furthermore, the carcass ply cords suffers repeated deformation during tire running, relative movement of respective filaments of the steel cords occur which cause localized abrasions therein. Due to repeated deformation, material fatigue is increasingly accumulated so that the strength of the cords is decreased and becomes unable to endure the internal tire pressure. This results in locally disconnected steel cords, destruction or separation of the carcass ply, and so on.
Thus, it is preferable to use textile cords. Textile materials generally used for the carcass ply cords are rayon, nylon (polyamide), polyester and so forth. Of these cords, the rayon cords are not desirable because of a significant reduction in a strength and deterioration due to moisture under high pressure and heavy duty conditions in long-term service. While the nylon cords usually employed for bias tires assure good cord strength, adhesion to the rubber compound and fatigue resistance, they are often likely to stretch due to tension caused by the internal pressure. When the nylon cords are used for radial tires, particularly when a steel belt is located circumferentially to prevent the tire from stretching in the radial direction of tire, stretching of the nylon cords results in an execessive increasee of tire width for high pressure, long-term-use in trucks and buses. In the case of a dual wheel drive, such an excessive increase of tire width permits the sidewalls of the tires to come in contact with each other. To prevent such side contact, tires are vulcanized with a mold having a somewhat narrower width, taking account of an appropriate tire width at the air pressure charging. However, since there is an appreciable difference in tire width between vulcanization-molding and at air pressure charging, separation occurs at the edge of the steel belt located at the shoulders of the tires which are subject to heavy deformation. Accordingly, the nylon cords are suitable for use in bias ply tires but not for radial tires or for use under high pressure in heavy duty trucks and buses. The polyester cords sharply contrast to the above described cords in that the polyester has satisfactory fatigue resistance against repeated deformation and is hardly affected by moisture. Further, the stretching of polyester cords is about half that of the nylon cords. The polyester cords are most preferably used in heavy duty radial tires.
Therefor, the present invention is directed to using polyester cords to form the carcass plies. Further, the present invention is directed to other factors for enhancing durability of the described polyester cords.
Careful attention is first directed to problems which are associated with the number of plies within the carcass. In the case that the carcass has a multiple ply structure, there will result differences in the tension of the inner and outer ply cords due to the internal air pressure. This causes further non-uniformity in strength, adhesion and other properties of the respective cords and thus accelerates a decline in strength, adhesion, etc., of the tire as a whole. It has been shown that strength and adhesion decline more and more when the vehicle travels with a low air pressure in the tire or with extremely heavy duty use and heavy deflection of the tire. The normal air pressure is not less than 6 Kg/cm.sup.2 for heavy duty truck and bus tires. The tire must be strong enough to endure such a relatively high pressure and the cord diameter should be adjusted correspondingly in the case where a reduced number of plies is used. Therefore, the cord diameter is the second problem which has to be addressed.
Moreover, in constructing the carcass, it is important how far respective ones of the ply cords are spaced from one another, particularly in the case of a small number of plies, though research activities so far have not been focused on that problem. In other words, for a small number of plies, a spacing between the cords, i.e., a center-to-center distance of the cords should be selected so as to maximize durability. The cord spacing is then the third issue addressed.
If the cord diameter is made greater with regard to the second problem addressed above, it becomes a requirement for preventing separation at the cutting edge of the ply cords turned up around the bead cores to have a precise cord spacing. When the carcass ply cords are made of high denier cord material, concentrated stress occurs at the cutting edge portion of the ply cords during tire running and causes the separation.